The invention relates to magnetic resonance imaging (MRI) pulse sequences and data processing.
MRI generally involves subjecting an object (e.g., a region of the human body) to a magnetic field, exciting nuclei in the object using an RF signal, and using the RF resonance signals emitted by the excited nuclei to generate an image of the object or a region thereof.
When excited, a nucleus at a point (x,y,z) resonates at a "Larmor" frequency f that depends on the strength of the local magnetic field B(x,y,z): EQU f(x,y,z)=.gamma.B(x,y,z) (1)
.gamma., the gyromagnetic ratio, is constant for a particular element in a particular compound. Because of the abundance of water in the human body, medical MRI typically images using hydrogen nuclei in water, which have a gyromagnetic ratio .gamma. of 4.258 kHz/Gauss (42.58 MHz/Tesla) Thus, in a 0.5 Tesla magnetic field, the Larmor resonant frequency f.sub.w of hydrogen nuclei in water is approximately 21.3 MHz.
Certain regions of tissue, such as a female breast, include relatively large volumes of intermixed fat and water. For a magnetic field of a given strength, the Larmor resonant frequency of a hydrogen nucleus in fat, f.sub.f, is 3.5 parts per million ("ppm") less than the resonant frequency of a hydrogen nucleus in water. Thus, a fat hydrogen nucleus in a 0.5 Tesla field resonates at a frequency that is 74.5 Hz lower than the resonant frequency of a water hydrogen nucleus.
As explained for example in U.S. Pat. No. 4,628,264, incorporated herein by reference in its entirety, a typical MRI system generates a magnetic field that varies in strength both spatially and temporally. The resonant frequency of a particular excited nucleus therefore depends on the location of that nucleus within the object being imaged. The MRI system exploits this dependence between resonant frequency and position to reconstruct an image from the sensed RF signal emitted by the excited nuclei.